JBL J820M Loudspeaker System (Sept. 1995)

JBL says that its new J Series of loudspeakers has been designed to provide
exceptionally accurate, uncolored, and balanced sound at affordable prices.
The line consists of five “bookshelf” speakers and two larger “tower” models,
plus a center-channel speaker specifically for home theater applications (the
others are equally usable in conventional music systems or home theater).

The J820M falls in the middle of the line in size and price. It is a two-way
system suitable for shelf or stand installation. Its 8-inch woofer, which
has a polymer-fiber cone, operates in a vented enclosure. The crossover,
at 3.5 kHz, is to a 14-millimeter (approximately ½-inch) titanium-dome tweeter.
The rated system response, at the —6-dB points, is 50 Hz to 20 kHz. Sensitivity
of the nominally 8-ohm system is specified as 90 dB, and it is recommended
for use with amplifiers rated between 10 and 125 watts output.

The speaker’s port opening is on its front panel, simplifying its installation
against a wall if desired. For listening and most measurements, however,
we placed the speakers on 25-inch stands several feet from the room walls.
The terminals, on the rear panel, are spring clips that accept stripped wire
ends. Unlike most such connectors, they are 3/4 inch apart and would also
accept dual banana-plug connectors (though not as easily as connectors designed
for that purpose).

Beginning with this speaker, we have slightly modified the room-response
measurement technique we have used for many years. Previously, a frequency-sweeping
signal generator and a synchronized chart recorder plotted the speaker’s
frequency response on graph paper. The purpose of this measurement was to
establish the effective frequency response of a pair of speakers in a fixed
“real-room” environment, under physical and acoustic conditions that are
maintained as nearly identical as possible for all speakers tested. That
process remains fundamentally unchanged, except that we now use our Audio
Precision System One to generate the test signals and digitally process the
output re turned by the Bruel & Kjaer 4133 measurement microphone. By
measuring the speakers’ room response under identical conditions in the same
environment with both types of instrumentation, we have established that
the results from the new procedure are very similar to those obtained with
the old, with the advantage that the new procedure is considerably faster
and more convenient to perform.

The room-response curve for the JBL J820M speakers (the average of the left
and right speakers’ outputs, smoothed and corrected for the known high-frequency
absorption characteristics of the room) was basically very similar for the
two measurement methods. It was very flat, with relatively little response
variation over most of the audio range (our averaging and smoothing process
minimizes the effects of the unavoidable room standing-wave patterns). Since
the smoothing function provided by the Audio Precision system is not identical
to that of our former instrumentation, there were numerous minor differences
between the results of the two tests, but their graphical characteristics
were strikingly similar.

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DIMENSIONS: 10 inches wide, 19¼ inches high, 10 inches deep

WEIGHT: 18 pounds

FINISH: Black woodgrain

PRICE: $300 a pair

MANUFACTURER: JBL, Dept. SR, 80 Crossways Park W., Woodbury, NY 11797

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Essentially, the response curve had two relatively flat plateaus, each covering
about half the spectrum. Be tween approximately 100 Hz and 1 kHz the smoothed
output varied over a ±1 -dB range. There was a distinct 4- dB drop between
1 and 1.5 kHz and a strikingly flat response (less than 1 dB overall variation)
from 1.5 to 10 kHz. Applying the high-frequency room correction (which we
have been using on all speaker measurements for about twenty-five years)
extended the flat high-frequency response to 20 kHz. It was interesting (and
gratifying) to find that our former measurement method produced the same
basic curve shape as our new procedure, though with slightly more variation
and departure from uniformity.

A close-miked woofer-response measurement indicated maximum out put at 135
Hz, falling by 6 dB at 80 Hz. The port radiation dominated at frequencies
below about 90 Hz. Such a measurement is inherently less ambiguous than the
room measurements, since it is essentially free of room interaction. Unfortunately,
it is not al ways easy to splice such a bass curve to a room curve in a meaningful
manner. Nevertheless, there was no doubt that the J820M easily met its response
specification of 50 Hz to 20 kHz at the —6-dB points. The actual lower limit
is hard to determine precisely because of room-boundary effects, but listening
tests indicated that it was somewhere between 40 and 50 Hz. Overall, the
J820M’s composite measured response was admirably smooth and extended for
a speaker of its size.

Quasi-anechoic MLS response measurements confirmed the impressive flatness
of the speaker’s output. The axial 2-meter response was flat within ±3 dB
from 300 Hz to 13 kHz, with a tweeter-resonance peak of 8 dB at 15 kHz. This
measurement also showed a pronounced notch at 3.8 kHz, apparently due to
the crossover (the notch did not appear in room measurements and was not
audible).

The J820M’s measured sensitivity with a 2.83-volt input was 93 dB sound-pressure
level (SPL), somewhat higher than rated. At a constant input of 2 volts,
equivalent to a 90-dB SPL output, distortion was between 0.7 and 1 percent
from 100 Hz to 2 kHz. It rose at lower frequencies, reaching 3.7 percent
at the rated low-frequency limit of 50 Hz. Below that point the output dropped
off fairly steeply, al though the distortion did not exceed 10 percent down
to 20 Hz.

Those results suggested that at reasonable average levels, driving the J820M
moderately hard at low-bass frequencies would not cause the sound to muddy
up with distortion, and that was confirmed in listening tests. The audible
bass output decreases as the frequency drops below 50 or 60 Hz but remains
reasonably clean in the process.

Taken as a whole, the JBL T820M is a very good small speaker. We listened
to it at length before making any measurements and were immediately impressed
by its smoothness and balance across the audible spectrum. As it happened,
the only other speakers available for comparison were considerably larger
and more expensive than the JBL’s, and correspondingly good in their performance.
A comparison between them and the J820M’s was inevitable, and it produced
surprising results.

In its balanced, uncolored sound, the J820M easily held its own in the comparison.
The larger speakers had a clear advantage in the low bass, but the J820M
had a more extended high- frequency response and greater perceived airiness.
Teamed with a sub- woofer, it could be a worthy competitor to the larger
system.

Bottom line, the JBL J820M is an extraordinarily fine-sounding speaker at
a bargain-basement price. It de serves a careful audition if you are in the
market for a compact, inexpensive, and thoroughly satisfying speaker.